Hovercraft and process of regulating air cushion

ABSTRACT

The surface effect vehicle comprises a plurality of lift fans for generating an air cushion, each of the fans having a drive with a motor and a regulating circuit with sensors and actuators for acting on the air cushion, each of the drives having an associated regulating circuit, the drive of the first one of the lift fans being operated as a master drive, while a rest of the lift fans being operated as follower drives. The process for regulating an air cushion in a surface effect vehicle with a plurality of lift fans whose drives have regulators and motors operated at a determined speed for generating an air cushion of a determined pressure or height and deliver corresponding output, a behavior of the lift fans conforming to a determined characteristic curve, the process comprises the steps of regulating every lift fan in such a way that one regulator serves as a master regulator which is followed by the rest of the regulators, so that the master regulator regulates a pressure or a height of the air cushion while the follower regulators regulate an output or a speed, where the output and volume flow or the speed of the master fan is used as a guidance variable.

BACKGROUND OF THE INVENTION

The present invention relates to a surface effect vehicle, in particulara waterborne vehicle, with a plurality of lift fans for generating anair cushion, each fan having a drive with a motor and a regulatingcircuit with corresponding sensors and actuators for acting on the aircushion. Further, the invention is directed to a process for regulatingan air cushion in a surface effect vehicle with a plurality of lift fanswhose drives have regulators and motors which are operated at adetermined speed for generating an air cushion of a determined pressureor height and deliver a corresponding output, the behavior of the liftfans conforming to a determined characteristic curve.

A surface effect vehicle of this type is known, for example, from DT 3638 785. The waterborne vehicle described therein has a plurality of liftfans for generating an air cushion, each fan having a drive with a motorand a regulating circuit which is linked with a computer and has sensorsfor influencing the air cushion.

The control can respond in various ways by suitable programmingdepending on the aim of optimization. However, this reference does notexplain the precise way in which the drives are to be regulated as afunction of the selected optimizing criterion.

The vertical fluctuations resulting from fluctuations in pressure cannotbe eliminated by the known regulating devices of lift fans, especiallywhen a plurality of lift fans are operated in parallel. This is becauseoscillations which can destroy the lift fans and other components occurconstantly during parallel operation of the lift fans.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide ahovercraft which avoids the disadvantages of the prior art.

More particularly, it is an object of the present invention to provide asurface effect vehicle and a process for operating lift fans whichenables economical operation and an improved performance of the vehicle.

This object is met according to the invention by a surface effectvehicle in which each drive has an associated regulating circuit and thedrive of a first lift fan is operated as a guiding drive or master drivewhile the rest of the lift fans are operated as follower drives.

The oscillations of the drives observed when lift fans are operated inparallel are prevented in this way. Every additional lift fan has itsown drive with associated regulating circuits and the drive of the firstlift fan is operated as a master drive, while the rest of the lift fansare operated as follower drives. The regulators are operated in such away that one or more of the other fans is so regulated, depending on theoutput of the master drive, that the ventilator speed is adjusted inproportion to the square of the difference of the total volume flow ofall parallel lift fans divided by the number of lift fans in paralleloperation minus the volume flow of the regulated fan. More simply, thevolume flow is measured and regulated. This essentially corresponds to aregulation of output, since the product of volume flow and air cushionpressure corresponds to output.

The measured output signal or speed signal or volume flow signal of thefirst lift fan is applied to the follower drives as a guide signal. Themaster regulating circuit is preferably a pressure regulating circuit orposition regulating circuit and the follower regulating circuits arepreferably speed regulating circuits or output regulating circuits orvolume flow regulating circuits. The signal for the volume flow ispreferably transmitted over a certain period of time which correspondsto a filtering. In so doing, the measured signal is not filtereddirectly, but rather the results of the regulating signal obtained aftersquaring are filtered.

Very brief disturbances, e.g., in the form of an individual wave, can beeliminated by regulation and ride quality can be increased when anadditional pressure regulating circuit having a time component smallerthan a time constant of a follower regulating circuit is associated withthe master or follower regulating circuit.

Especially in view of the fact that the lift fan comprises aerodynamicelements, in particular blades with servoactuators, the ventilatorcharacteristic curve can be changed temporarily in such a way that theventilators temporarily displace more volume or less volume without anappreciable change in speed caused by a change in the characteristiccurve.

Also, the characteristic curve of the ventilator is changed in that thelift fan has a reversible-flow duct which is closed during normaloperation and constructed so as to connect the pressure side with theinduction side and is opened, especially automatically, preferably byadjusting a flap, which allows a fast response to external disturbancesin an economical construction.

The reversible-flow flaps can be pushed into their closed position,e.g., by a pretensioned spring. When a given cushion pressure isexceeded, the pretensioning is overcome and the flap opens. It closesagain when the pressure drops. The actuating forces can be influenced inan advantageous manner by a systematic support of the flap.

To be particularly effective, the reversible-flow duct has a dosingmechanism whose drive is connected as an actuator in the continuingpressure regulating circuit. It is particularly advantageous to use thesquare of the defined regulating variable to control aerodynamicelements such as the guide blades or reversible-flow ducts.

Another construction of the invention provides an operating pointdetection device with an evaluating device for signalling an unstableoperating point, preferably with automatic resetting to a stableoperating point, and/or a cut-off device. This prevents unstableoperating points, which are also uneconomical. In fact, it has beenobserved that air flows backwards through individual lift fans in partover long time periods and these additional losses must be compensatedby the rest of the ventilators with a correspondingly high volume flow.In such cases, a ventilator operating unstably can be switched offautomatically by suitable automatic detection of the operating pointwhich can be realized, e.g., by calculating the quotient of the volumeflow and speed. This considerably improves operation. In order to switchoff, the ventilator must be outfitted with non-return valves forinstance. A further possibility consists in using the results of themonitoring of the operating point to generate an alarm signal to alertthe operator of the vehicle so that appropriate steps can be taken.Finally, it is possible to reset the operating point of the ventilatorin question to a stable range by a forced increase in speed. The powerlosses resulting from reverse flow which are brought about by the escapeof previously compressed air from the air cushion are accordinglyprevented. Another disadvantage of such uncontrolled pressurefluctuations in the air cushion is that they lead to unpleasant motionin the ship which limits the useful possibilities of the vehicle. Thelift fans are regulated according to the invention in such a way thatthey operate in the stable range of the characteristic curve with onlybrief exceptions. When one or more lift fans move into the vicinity ofthe peak of the characteristic curve, this means that the pressure istoo high. This causes a surface effect vehicle to be lifted tounnecessarily high. In conventional vehicles, this results in a periodicblow-off of the excess air which is followed by a rapid fall of thevehicle and has a negative impact on the ride quality of the vehicle.

Particularly advantageous operating behavior results when the regulatorin the pressure regulating circuit is constructed so as to act upon theoutput or speed of the lift fan based on a rule which provides for achange in speed (Δn) which is proportional in terms of quantity to thesquare root of the reference-to-actual deviation of the measureddifferential pressure between the cushion (Δp_(actual)) and atmosphericpressure and the reference differential pressure (Δp_(reference)). In sodoing, the regulator preferably adjusts the speed or outputproportionally to the root of the differential pressure between areference differential pressure which is set from the bridge and theactual differential pressure measured in the cushion according to thefollowing formula: ##EQU1## Further advantageous constructions of thevehicle are described in claims 8 to 10.

The object of the invention is also met in a process of the generic typein that every lift fan is regulated in such a way that one regulatorserves as a master regulator which is followed by the rest of theregulators, so that the guiding master regulator preferably regulatesthe pressure or the height of the air cushion and the followerregulators regulate the output or speed, where the output and volumeflow or the speed of the master fan is used as a guidance variable.

Because the guiding master regulator regulates pressure or height andthe follower regulators regulate the output or speed, where the outputor volume flow or speed of the master fan is used as a guidancevariable, the speed of the satellite fans is stepped up or stepped downin such a way that all fans have identical volume flow or at leastidentical output. All fans then operate at approximately the sameoperating point. The regulating system according to the invention inwhich only one lift fan is set at a reference speed, while the otherlift fans are adjusted by a follower regulation to maintain the samevolume flow, ensures that all lift fans contribute in equal measure tothe production of the leakage volume flow in the air cushion. Thisleakage volume flow is predetermined on the basis of the loading stateand possibly other parameters such as vehicle speed, sea state, etc.This simplifies adaptation to the required cushion pressure and preventsexcessive reciprocal influence of the lift fans and relativeoscillations. Therefore, there is no negative flow through any of thefans for an extended period. In this way, the lift fans are reliablymaintained in the range of stationary operating behavior and asatisfactory economical operation is ensured.

When they are acted upon individually by bypass flaps for the purpose ofgenerating reverse flow, the lift fans can also be influencedaerodynamically depending on the pressure of the lift fan, preferably bygenerating a reverse flow.

If one or more fans have non-return valves, unwanted reverse flows areprevented as soon as a fan is switched off automatically or manually.

The novel features which are considered as characteristic for theinvention are set forth in particular in the appended claims. Theinvention itself, however, both as to its construction and its method ofoperation, together with additional objects and advantages thereof, willbe best understood from the following description of specificembodiments when read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the equivalent diagram of the air cushion of a surfaceeffect vehicle with only one lift fan and its slow regulation of speed;

FIG. 2 shows the characteristic curve of a lift fan;

FIG. 3 shows the equivalent diagram of a surface effect vehicle with amaster lift fan and a satellite lift fan;

FIG. 4 shows a family of characteristic curves for a plurality of liftfans in parallel operation in the stationary range;

FIG. 5 shows a side view of a lift fan with partially exposed housing;

FIG. 6 shows a top view of the lift fan with partially exposed housing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, the system boundary of an air cushion 1 is representedschematically by the rectangle. In the air cushion 1, a determinedvolume flow 3 is transported in the direction of the arrow by the liftfan 2. A leakage volume flow 4, represented by an arrow, escapes fromthe air cushion 1 and counterbalances the feed volume flow 3 over thelong term. Short-term differences between the feed volume flow 3 andleakage volume flow 4 lead to changes in the air cushion 1 with respectto its pressure difference Δp and volume V state variables. Therefore,given constant surface area of the air cushion, changes in volume can beobserved as changes in the height of the air cushion. Interferencevariables such as travel speed and wave height are manifested as changesin the leakage volume flow 4.

The cushion pressure difference Δp is detected as a measurement quantityof the air cushion and the feed volume flow 3 is preferably detected asthe measurement quantity for the lift fan 2. The product gives theaerodynamic output of the lift fan.

The master fan 2 is driven by the motor 5 which is acted upon by theregulator 6. The signal from a pressure sensor 7 and, via line 8, thesignal from a reference value transmitter 9 installed in the bridge ofthe vehicle are applied to the regulator 6.

FIG. 2 shows a typical characteristic curve for the radial ventilatorused as lift fan. The characteristic curve is valid for a determinedspeed. For every speed, there is an outlet pressure range ΔP_(2max) toΔP_(2min) plotted as pressure on the ordinate. In this case, Δ signifiesa differential pressure. At a specific volume flow V, the characteristiccurve runs in the direction of the abscissa through a peak with pressureΔP_(2max). Accordingly, for pressures between ΔP_(2min) and ΔP_(2max)there are, in each instance, two theoretical operating points A, B atthe same pressure ΔP_(2'), although with very different volume flowsV_(A) and V_(B). Problem-free parallel operation of two ventilatorswould not be possible with this characteristic curve in the pressurerange of Δp_(2max) to Δp_(2min) without taking special steps. Forexample, in the event that a lift fan operates at operating point B, thefirst lift fan moves farther to the left on the characteristic curve tothe smaller volume flow and greater ΔP2 in the new operating point whenthe cushion pressure increases due to external influences such as seastate. A second lift fan is pushed toward a lower volume flow by theincreasing air cushion pressure until its pressure corresponds to theair cushion pressure at operating point E'. As will be seen from thedrawing, the displaced volume can be negative, i.e., air flows backwardthrough the ventilator at -V2, which is designated as overblowing.

This operating point B and, in general, every operating point to theleft of the peak is undesirable, since it can result in constantoscillations which can destroy the lift fan and other components.

FIG. 3 shows a surface effect vehicle according to the invention withtwo lift fans in parallel operation. To distinguish the parts of the twofans having identical functions, those parts associated with the masterfan receive reference numbers with "L" and those associated with thefollower fans receive reference numbers with "F". The follower fan alsohas a regulator 6 and a motor 5 acted upon by the latter. Themeasurement signal from a volume flow sensor 10 and the signal from avolume flow sensor 10 are applied to the regulator as input quantities.

According to the invention, one of the parallel lift fans is connectedas a so-called master fan which is adjusted manually from the bridge bytransmitters 9 to an air flow or speed corresponding to the desiredpressure. The speed is stepped up to the required value by a speedgovernor 5. This regulation produces an increase in speed when thepressure drops relative to the predetermined value. Conversely, anincrease in pressure leads to a reduction in speed. The regulation is sodesigned that the speed changes by the square of the pressure differenceand is adjusted asymptotically. The cushion pressure is measured by thepressure sensor 7. The signal transmitted over an adjustable time periodis utilized for comparison with the reference pressure. The otherparallel cushion fan or fans has/have a speed regulation which uses thedifference in the volume flows of the master fan and individual volumeflow, or possibly also the output differential, as an input quantity.These input quantities are constantly detected by the volume flowtransmitters 10_(L) and 10_(F) using measurement techniques. The speedof the follower satellite fans is then stepped up or stepped down sothat all fans produce the same volume flow 3F or at least the same waveoutput. In this case, all of them are working at approximately the sameoperating point.

Thus, only one lift fan is regulated at a reference speed via thecushion pressure, while the other lift fans are adjusted to maintain thesame volume flow by a follower regulation. This ensures that all liftfans contribute approximately equally in producing the leakage volumeflow 4 in the air cushion 1. The leakage volume flow is predetermined asa function of the loading state and, as the case may be, on otherparameters such as vehicle speed, sea state, etc.

FIG. 4 shows a family of characteristic curves for a lift fan atdifferent speeds to illustrate the slow regulating system. In thestationary state, for example, as will be seen from FIG. 4, the masterfan works at operating point A which is the intersection of thecharacteristic curve at normal speed n_(n) and the resistancecharacteristic curve a of the air cushion. If the cushion pressureshould drop, for instance, when traveling on sea because more airescapes from the air cushion due to increased leakage volume flow, theresistance characteristic curve b is adjusted. Consequently, theoperating point of the master fan moves from A to C at constant speedand the volume flow increases from V_(A) to V_(C). At the same time, thecushion pressure drops and the immersion depth of the vehicle increases.The speed regulation which is now initiated increases the speed to n₁ sothat the originally desired cushion pressure is restored at point D. Thevolume flow increases from V_(C) to V_(D). However, this increase neednot be coped with exclusively by the master fan. The satelliteventilators follow along via the load distribution control and take overtheir share of the required volume flow increase.

On the other hand, in the event of a decrease in the mean leakage volumeflow, e.g., corresponding to the throttle curve C, the cushion pressurewould increase and the new operating pressure E would be adjusted.However, the overpressure, in proportion to the given referencepressure, decreases the speed of the master fan, followed by thesatellite fans, to the new speed n₂ at operating point F. The regulatingtime constants are fallible, so that the mean cushion pressure remainsas constant as possible. In this case, it may be advisable that the timeconstant of the master fan regulation is greater than that of thesatellite regulators so that the latter lag only slightly behind themaster fan.

Rapid variations in load, e.g., at the wave frequency, will hardlyaffect the speed due to the large rotating masses of the lift fan anddriving motor. Since the speed regulation system is controlled by a meanpressure value over an adjustable time period, it also does not receiveany control pulses. A typical regulating time constant is roughly 30 to60 seconds.

If the operating point should move into the region to the left of thepeak of the characteristic curve under the influence of rapid reductionsin leakage flow, it is possible to carry out additional regulatingsteps. These steps are so designed that the volume flow displaced in theair cushion is reduced very quickly to prevent the peak from beingexceeded.

To this end, a bypass regulation is provided according to the inventionto return the lift fan or lift fans to the region to the fight of thepeak of the characteristic curve. The bypass regulation has theadvantage that no additional blow-off ducts need be provided, i.e., theyare integrated directly in the lift fan. Accordingly, the bypassregulation effects a change in the characteristic curve of the lift fan.This regulation according to the invention takes effect more rapidlybecause of the short distances and smaller moved masses. At low volumeflow, the operating point moves from A to E to F as the leakageresistance increases as shown in FIG. 4. When the resistance curve jumpsback to a, the operating point creeps from F to G and slowly back towardA. In this case, the bypass regulation does not come into play. However,if the leakage resistance climbs to throttle curve d, the new unstableoperating point H is adjusted. Before reaching the point when the slowspeed regulation of the ventilator can respond, the bypass regulationtakes effect. By recirculating a portion of the air, shown here for avolume flow V_(rezi), the new line n_(nrezi) occurs. The intersectionwith the throttle curve d gives the operating point K which hardlydiffers from H, but which is stable. When the resistance continues toincrease, the ventilator is not overblown.

According to the invention, the bypass is controlled in such a way thatit opens above a given limiting pressure. For this purpose, for example,the control can respond to the volume flow, dynamic pressure or cushionpressure.

Since the pressure compensation in the air cushion takes a finite periodof time because of the necessary displacement of air masses, it isparticularly advantageous to effect the pressure decrease directly atthe lift fan in order to maintain the lift fan in the stable operatingregion and prevent negative flow of air.

According to the invention, if the bypass regulation responds frequentlyas detected preferably by a counting instrument, the bridge is alertedwhen this frequency exceeds a determined quantity. The ventilatorsdisplace substantially more air than is necessary. Accordingly, one ormore fans can be switched off manually or automatically at determinedintervals. In so doing, the flow-in opening of the fans is closed.Non-return valves are advantageously provided for closing, since theyrequire no additional actuating drives.

It will be observed from inspection of conventional surface effectvehicles that the lift fans are usually designed for the heaviest seaswith high leakage flow losses. For a large part of the time, however,they are operated in good weather with low leakage losses. They are thenforcibly operated close to peak with frequent slipping into the overflowrange. Riding comfort is reduced and energy consumption is higher thanwhen a smaller number of ventilators are operated. The same effect isnot achieved by stepping down the speed, since the change-over point isonly displaced slightly.

In heavy seas and accordingly with sharply varying leakage flows, it maybe advantageous to operate the ventilators at high speed and to keep thebypass flaps open partially at all times. When the leakage flow dropsand pressure increases, more air flows back through the bypass so thatthe ship is stabilized.

It is a different matter if the pressure in the air cushion drops whenthe vehicle is exchanged, since more air can escape. The volume flowdelivered in the air cushion must now be increased quickly, i.e., thebypass regulation must be closed again quickly.

Special requirements are imposed on the regulation of lift fans byunstable behavior of the lift fans which occurs when traveling over seadue to rapid changes in the air cushion. The characteristic curve thenexhibits a frequency-dependent hysteresis.

FIG. 5 shows a fan, according to the invention, with a bypass forrecirculating a partial volume flow. It has a housing 11 with an axiallyaligned suction or intake pipe 12 and a tangentially arranged diffusor13 through which the air is expelled. Part of the housing wall is notshown as indicated by the broken line 14, so that the portion of theintake pipe 12 projecting into the housing 11 is visible. A flap 15 isprovided in this portion and can be opened by swiveling an actuatinglever 16 about axis 17 (FIG. 6). The flap is shown in its open positionby the dashed line. In this position, air can be guided out of thepressure region of the lift fan according to arrow 18 back into thesuction region of the intake pipe 12.

The actuating lever 16 is connected with a suitable servodrive which iscontrolled in turn by a regulator as described above.

For certain applications it may be sufficient to keep the flap closedmerely by pretensioning to a certain extent so that it opensautomatically when exceeding a pressure which overcomes thepretensioning.

In this way, a surface effect vehicle having improved performance andeconomical operation is provided.

It will be understood that each of the elements described above, or twoor more together, may also find a useful application in other types ofconstructions differing from the types described above.

While the invention has been illustrated and described as embodied in ahovercraft and process of regulating air cushion, it is not intended tobe limited to the details shown, since various modifications andstructural changes may be made without departing in any way from thespirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed as new and desired to be protected by Letters Patent isset forth in the appended claims:
 1. A surface effect vehicle,comprising a plurality of lift fans for generating an air cushion, eachof said fans having a drive with a motor and a regulating circuit withsensors and actuators for acting on the air cushion, each of said driveshaving an associated regulating circuit, said drive of the first one ofsaid lift fans being operated as a master drive, while a rest of saidlift fans being operated as follower drives which receive a signal fromsaid master drive.
 2. A surface effect vehicle as defined in claim 1,wherein said guiding drive is a drive.
 3. A surface effect vehicle asdefined in claim 1 it and further comprising means for producing asignal applied to said follower drives as a guide signal, saidregulating circuits include a master regulating circuit associated withsaid master drive and follower regulating circuits associated with saidfollower drives.
 4. A surface effect vehicle as defined in claim 3, andfurther comprising means for producing a signal selected from the groupconsisting of a measured output signal, a speed signal and a volume flowsignal.
 5. A surface effect vehicle as defined in claim 3, wherein saidmaster regulating circuit is formed as a circuit selected from the groupconsisting of a pressure regulating circuit and a position regulatingcircuit.
 6. A surface effect vehicle as defined in claim 3, wherein eachof said follower regulating circuits is a circuit selected from thegroup consisting a speed regulating circuit, an output regulatingcircuit and a volume flow regulating circuit.
 7. A process forregulating an air cushion in a surface effect vehicle with a pluralityof lift fans whose drives have regulators and motors operated at apredetermined speed for generating an air cushion of a predeterminedparameter selected from the group consisting of a pressure and a heightand deliver corresponding output, a behavior of the lift fans conformingto a predetermined characteristic curve, the process comprising thesteps of regulating every lift fan in such a way that one regulatorserves as a master regulator which is followed by the rest of theregulators, so that the master regulator regulates a parameter selectedfrom the group consisting of a pressure and a height of the air cushionwhile the follower regulators receive a signal from the master regulatorand regulate an output or a speed, where the output and volume flow orthe speed of the master fan is used as a guidance variable.
 8. Theprocess as defined in claim 7, and further comprising the step of actingupon the lift fans aerodynamically during the regulating depending uponpressure.
 9. The process as defined in claim 7, wherein said step ofacting includes acting on the lift fans by generating a reverse flow.10. The process as defined in claim 7; and further comprising the stepsof monitoring an operating point of the lift fan during the regulatingand generating an alarm signal automatically when an unstable operatingpoint is detected.
 11. The process as defined in claim 7; and furthercomprising the steps of monitoring an operating point of the lift fanduring the regulating; and stopping the lift fan automatically when anunstable operating point is detected.
 12. The process as defined inclaim 7; and further comprising the steps of detecting a frequency ofreverse flow events during the regulating; and effecting regulation witha hysteresis which is adjusted in accordance to the frequency of thereverse flow events determined.
 13. A surface effect vehicle, comprisinga plurality of lift fans for generating an air cushion, each of saidfans having a drive with a motor and a regulating circuit with sensorsand actuators for acting on the air cushion, each of said drives havingan associated regulating circuit, said drive of the first one of saidlift fans being operated as a master drive, while a rest of said liftfans being operated as follower drives; and further an additionalpressure regulating circuit having a time constant of at least one ofsaid master regulating circuit and said follower regulating circuit,said additional pressure regulating circuit being associated with atleast one of said master regulating circuit and said follower regulatingcircuit.
 14. A surface effect vehicle, comprising a plurality of liftfans for generating an air cushion, each of said fans having a drivewith a motor and a regulating circuit with sensors and actuators foracting on the air cushion, each of said drives having an associatedregulating circuit, said drive of the first one of said lift fans beingoperated as a master drive, while a rest of said lift fans beingoperated as follower drives, said lift fans having a duct which isclosed during normal operation and formed so as to connect a pressureside with an induction side and is opened automatically; and furthercomprising a flap operative for automatically opening said reversibleflow duct.
 15. A surface effect vehicle as defined in claim 14, whereinsaid duct is provided with a closing mechanism including a drive.
 16. Asurface effect vehicle, comprising a plurality of lift fans forgenerating an air cushion, each of said fans having a drive with a motorand a regulating circuit with sensors and actuators for acting on theair cushion, each of said drives having an associated regulatingcircuit, said drive of the first one of said lift fans being operated asa master drive, while a rest of said lift fans being operated asfollower drives; and an operating point detection device with anevaluating device for signaling an unstable operating point.
 17. Asurface effect vehicle as defined in claim 16; and further comprisingmeans for automatically resetting to a stable operating point inresponse to the signalling an unstable operation point.
 18. A surfaceeffect vehicle as defined in claim 16; and further comprising a cut-offdevice operative in response to the signalling of an unstable operatingpoint.
 19. A surface effect vehicle, comprising a plurality of lift fansfor generating an air cushion, each of said fans having a drive with amotor and a regulating circuit with sensors and actuators for acting onthe air cushion, each of said drives having an associated regulatingcircuit, said drive of the first one of said lift fans being operated asa master drive, while a rest of said lift fans being operated asfollower drives, said lift fan being arranged diffusor in a housing,said lift fans having a reversible-flow duct formed as a dividing flapbetween said intake pipe and said diffusor.
 20. A surface effectvehicle, comprising a plurality of lift fans for generating an aircushion, each of said fans having a drive with a motor and a regulatingcircuit with sensors and actuators for acting on the air cushion, eachof said drives having an associated regulating circuit, said drive ofthe first one of said lift fans being operated as a master, while a restof said lift fans being operated as follower drives; means for producinga signal applied to said follower drives as a guide signal, saidregulating circuits include a master regulating circuit associated withsaid master drive and follower regulating circuits associated with saidfollower drives; said master circuit being a pressure regulating circuitprovided with a regulator, said regulator being formed so as to act uponone of an output and a speed of said lift fan based on a rule whichprovides for a change in speed which is proportional in terms ofquantity to a square root of a reference-to-actual value deviation of ameasured differential pressure between the air cushion and atmosphericpressure and a reference differential pressure.